(1) Field of the Invention
The present invention relates to a biodegradable chemically modified plasticized starch (CMPS) composition, and a chemically modified starch-nanoclay CMPS composition, and to a process for the preparation thereof. The invention preferably relates to a twin screw extrusion process for manufacturing the chemically modified starch composition by reacting starch with a dibasic acid or anhydride of the acid as a modifier in the presence of plasticizer and an optional free radical initiator for the acid or anhydride to provide the plasticized starch compositions which have improved processability and reduced viscosity, and is particularly useful when combined with other thermoplastic polymers.
(2) Description of Related Art
In the last two decades, considerable effort has gone into the development of biodegradable polymers and blends using starch. The processing characteristics and mechanical properties of unmodified starch polymers are very poor compared to typical synthetic polymers.
The prior developments in the area of biodegradable materials from starch involve the use of an external plasticizer to solvate the starch granules and impart melt processability and enhance the room-temperature flexibility of the final product. Some of the earliest efforts on such materials were discussed in U.S. Pat. No. 3,312,641 (A. E. Staley Mfg. Co.) and Canadian Patent Appl. No. 838,572 (USDA, Lincoln, Nebr.). The subject of these patents was the extrusion of amylose primarily using water as a plasticizer. In addition, poly (vinyl alcohol), glycerol and other related materials were also incorporated in the system. Cast films from water-based dispersions of starch, glycerol and other polymers were evaluated in U.S. Pat. Nos. 3,949,145, 4,133,784, and 4,337,181 (USDA, Peoria, Ill.). More recently, extruded compositions using water were discussed in Eur. Pat. Appl. EP 282,451, 304,401, 327,505, 404,723, 404,728, and 408,503 (Warner Lambert Co.); water-free systems using glycerol were discussed in PCT Int. Appl. WO 90/05,161 (Tomka); and formulations using both water and other plasticizers like glycerol were the subject of PCT Int. Appl. WO 92/19680 (Novamont S.p.a.), Eur. Pat. Appl. EP 400,531 (Butterfly S.r.l.), and PCT Int. Appl. WO 90/14,388 (Agri-Tech Industries, Inc.).
WO92/19680 (Novamont) describes the use of undried starch (about 12% moisture) in biodegradable polymeric compositions. These compositions produce films whose properties are sensitive to moisture. WO93/00116 (P&G) describes plastic compositions, which use “destructurized” starch, a form where the crystallinity of the starch has been destroyed by heating. The term “destructurized starch” refers to starch in which the granular, crystalline structure is destroyed resulting in the formation of a homogeneous, amorphous material that melts and flows like a thermoplastic. The “destructurization” process is accomplished by heating starch granules in the presence of water at elevated temperatures and pressures in a closed volume like an extruder. The higher the amount of water used, the lower the temperature needed for “destructurization” to be completed. Typical compositions for the “destructurization” processes include extruding starch with about 15–20% water by weight along with other additives. It has been argued that the concept of “destructurization” is just a new name ascribed to an old concept of gelatinization and/or plasticization of starch that first originated in the food industry. The term gelatinization refers to the process of disruption of the granular structure of starch by heating a starch suspension at temperatures in the range of 50°–80° C. Traditional processing in the food industry involves further heating and mixing of starches to result in the formation of a homogeneous plasticized mass as discussed by Shogren, R. L., et al., Starch/Starke 45, 276–280 (1993). The plasticization of starch using water and glycerol has been reported as early as 1947 for pharmaceutical applications (“The Pharmacopoeia of the United States of America”, XIII, p. 513–514, 1947).
The principal difference between the various applications has to do with the ratios of starch and plasticizer (water, glycerol or both) used in the composition. This leads to differences in the rheological and final characteristics of each composition that is suited to the specific application. A high proportion of plasticizer (70–90% by weight) results in a jelly-like material. Medium plasticizer contents (40–70% by weight) are more suited for food applications that have good elasticity combined with reasonable rigidity to be able to form and shape various products. Low plasticizer contents (10–40% by weight) are appropriate for plastics applications that require greater rigidity and better mechanical strength characteristics. More recently, plasticization of starch in the absence of water, using polyhydric alcohols like glycerol and sorbitol has been reported. The term plasticization in the classical sense as used in the polymer industry involves transformation of a rigid, crystalline material into a pliable, less crystalline (if not amorphous) material with a lowered glass transition temperature. Japanese Patent No. 04-48,851 discloses the manufacture of biodegradable sheets based upon mixtures of starch and biodegradable polyesters derived from reactions of diols and acids. The starch has normal moisture content of about 10 to 12% by weight.
In a typical plasticized system with starch, the diffusion of plasticizer out of the product when exposed to low humidity conditions and diffusion of water into the product under high humidity conditions is an inevitable result. This causes embrittlement of the product due to loss of plasticizer (low humidity) and problems associated with retention of product shape, texture, and form due to excess absorbed water (high humidity). These effects are detrimental especially when water is used as a plasticizer, but are prevalent even in non-water based starch formulations that incorporate hygroscopic plasticizers. In general, due to the poor durability of plasticized starch upon exposure to different environments (due to its hydrophilic nature) there is likely to be very little commercial use of plasticized native or unmodified starch by itself.
Physical or chemical modifications of the starch are a viable alternative to solving some of these problems. Physical modifications include coating the starch granules with hydrophobic sizing agents and binders similar to those used in the paper industry, like rosin, proteins, like soy proteins and silanes or physically coating the end product with hydrophobic materials like low molecular weight waxes, and natural resins (zein, rosin, shellac and the like) and higher molecular weight non-polar polymers. Cross-linking of the starch granules or plasticized starch is another physical modification route to improving the hydrophobicity of starch-based materials. Chemical modifications include grafting reactions and non-degradative substitution of the hydroxyls on the starch with functional groups like esters, ethers, isocyanates and the like. Cyclic dibasic acid anhydrides such as succinic anhydride yield starch esters containing a free carboxylate group that increases the water-holding capacity of the product (U.S. Pat. No. 2,461,139; 1949). The same chemistry can be carried out with maleic anhydride to yield the maleate half ester. Treatment of a starch suspension with a cyclic dicarboxylic acid anhydride containing a hydrophobic substituent group yields products with emulsion stabilizing properties (U.S. Pat. No. 2,661,349; 1953). These starch esters have value as dispersants in coatings replacing commercially available paint containing dispersants. U.S. Pat. No. 3,732,207 discusses the production of starch esters by heating starch with maleic or succinic anhydride at 100–155° C. in a batch process with reaction times of 10 hours in the presence of 0.5–15% moisture to give a product with degree of substitution (DS) 0.02 to 0.04.
Tomasik et al., Starch 47 96 (1995) reacted corn starches in extruders containing varying amounts of moisture (18, 20 and 30%) with succinic, maleic and phthalic anhydrides to form starch esters. Carbonate buffer, either pH 8 or pH 9 was added as a medium during extrusion. It is demonstrated that extrusion of starch with cyclic anhydrides in alkaline medium presents a facile method of preparation of anionic starches of hydrophobic character based. However, this method has several serious limitations resulting from the chemistry and the extrusion process. For example, it is extremely difficult to extrude these samples at temperatures of around 130° C. due to the resultant high viscosity of the starch melt. Also, the use of water is extremely problematic in processing and quality of the final product.
Related prior art is described in:    (1) Ramani Narayan, Steven Bloembergen and Amit Lathia, A Method of Preparing Biodegradable Modified-Starch Moldable Products and Films, U.S. Pat. No. 5,869,647, Feb. 9, 1999, July 1993;    (2) Narayan, R., Biodegradable Multi-Component Polymeric Materials Based on Unmodified Starch-Like Polysaccharides, U.S. Pat. No. 5,500,465, Oct. 31, 1995;    (3) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,540,929, Jul. 30, 1996;    (4) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,578,691, Nov. 26, 1996;    (5) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,616,671, Apr. 1, 1997;    (6) U.S. patent application co-filed with the present application.